1. Field of the Invention
The present invention relates to a lead acid battery, and more particularly, to an improved valve regulated lead acid battery having lightly gelled electrolyte.
2. Description of the Prior Art
Lead acid batteries, developed in the late 1800s, were the first commercially practical batteries. Rechargeable lead acid batteries have been available since the 1950s and have become the most widely used type of batteries in the world. The following equation shows the discharge chemical reaction in a lead acid cell:PbO2+Pb+2H2SO4→2PbSO4+2H2O
Lead acid batteries remain popular because they can produce high or low currents over a wide range of temperatures and have good shelf life and life cycles. Furthermore, they are relatively inexpensive to manufacture and purchase. Another advantage of the lead acid batteries is they come in all manner of shapes and sizes, from household batteries to large batteries for use in submarines.
Valve regulated lead acid (VRLA) batteries are one kind of the lead acid batteries. A VRLA battery comprises positive plates, negative plates, separators, electrolyte, and a container with one-way valve installed to prevent external gases from entering the battery where the oxygen would react with the plates then causing internal discharge and allow gas to vent from the interior of the battery when a certain internal pressure is exceeded. Please refer to FIG. 1. FIG. 1 is a sectional view of a conventional lead acid battery. As shown in FIG. 1, a lead acid battery 10 comprises a plurality of alternating positive plates 12 and negative plates 14 with separators 16 sandwiched in between the adjacent plates and electrolyte (not shown). The positive plates 12 and the negative plates 14 are made by a formation process which converts lead grids pasted with positive active material such as lead oxide, and with negative active material such as lead powder into the positive plates 12 and negative plates 14 respectively.
In a VRLA battery, the electrolyte is generally immobilized. The immobilization of the electrolyte in a VRLA battery makes it possible for the gases generated at one electrode have an access to the other electrode during charging. As a result the oxygen gas is able to travel inside the battery, and is reduced at the surface of the negative plates and returned to the electrolyte of the battery. Additionally, the immobilization of the electrolyte also prevents the liquid electrolyte from the danger of spilling which is highly acidic and corrosive.
There are two mainly well-known categories of methods to immobilize the electrolyte in a lead acid battery: by absorption or gelling. U.S. Pat. No. 4,871,428 discloses a glass mat separator made from glass microfibers and used to immobilize the liquid electrolyte by absorption, and firmly and intimately contact the plates, thereby assuring good initial electrolyte-plate contact and hence higher energy efficiency. However, the intimate contact between the glass mat separator and the plate surfaces easily forms a needle-like dendrite at the negative plate then growing into a tunnel-like passage way contacting the positive plate through the adjacent separator pore, thus finally shorting the battery.
U.S. Pat. No. 4,317,872 discloses an alternative method to immobilize the electrolyte by reacting sulfuric acid with silica particles to form a gel electrolyte. However, the gel electrolyte suffers various inferior electrical properties such as higher internal electrical resistance, lower energy capacity, and reduced cycling characteristics. In addition, the gel electrolyte has a tendency to shrink so that the contact between the gel electrolyte and the active material on the plates may be interrupted. Furthermore, the initial high viscosity of the gel also presents a challenge of being difficult to fill a battery container, and being not able to efficiently saturate the pores of the plates.
U.S. Pat. No. 4,889,778 discloses a thixotropic gel electrolyte consisting of a blend of an aqueous colloidal dispersion of about thirty percent (30%) by weight alkali metal polysilica having the formula [Y2O].x[SiO2].nH2O with sulfuric acid in a ratio of 1:3 to 1:6 by volume. The x ranges from about 20 to 350, Y is an alkali metal and n is the number of moles of water. Finally, U.S. Pat. Publication No. 2005/0042512 discloses a lead acid battery electrolyte comprises sulfuric acid having a specific gravity between 1.250 and 1.280 and silica particles in a concentration between 2 and 15 weight percent. At least 10% by weight of the silica particles are from a never-dried precipitated silica slurry.
While the prior art describes the improved gel electrolyte preparation methods facilitating filling of the container, the gel electrolyte based lead acid battery still has general inferior electrical properties than those made with ordinary liquid electrolyte such as growing internal electrical resistance during discharge, lower energy capacity caused by reduced amount of active electrolyte material, and potential shrinkage problem caused by high silica content, etc.